This invention relates to a protect network device (hereinafter referred to as "protection device") for use in a communication system adapted to be provided in a communication interface between the subscriber side such as telephone lines and an apparatus to be protected (hereinafter referred to as "the subscriber line interface circuit" or simply as "the circuit side") such that the latter can be protected against an abnormally large voltage or current caused, for example, by lightning or contact with a power line. In particular, this invention relates to such a protection device which uses at least a thermistor with positive temperature characteristic (hereinafter referred to as a PTC thermistor) or a resistor.
Japanese Patent Publication Tokko 3-5135 disclosed a protection device for a communication system which includes a protection circuit illustrated in FIG. 9 and is currently in use as a communication safety device. This protection circuit is characterized as having arresters 1 and 2 which are connected to each other in series and between the input terminals on the subscriber side of lines A and B, a connecting point therebetween being grounded. A series connection of two varistors 3 and 4 is connected between the lines A and B of the output terminal on the side of the subscriber line interface circuit, or the circuit side, a connecting point therebetween being also grounded. A series connection of a PTC thermistor 5 and a thick-film resistor 7 is connected to the line A, and another series connection of another PTC thermistor 6 and another thick-film resistor 8 is connected to the line B between the arresters 1 and 2 and the varistors 3 and 4 as shown.
In this protection circuit, the PTC thermistors 5 and 6 and the thick-film resistors 7 and 8 are each adapted to function as a protective element against overcurrent, but the PTC thermistors 5 and 6, in particular, have the disadvantage when used for this purpose in that their resistance values have large deviations. For this reason, the thick-film resistors 7 and 8 are adapted to be trimmed such that the impedance imbalance between the lines A and B can be reduced by controlling the resistance values of the series connections of the PTC thermistors 5 and 6 and the thick-film resistors 7 and 8 to the level of accuracy of about .+-.1%.
With a protection device thus formed, a surge by lightning entering the communication lines is absorbed by the arresters 1 and 2 at the first stage. A lightning surge which failed to be absorbed by the arresters 1 and 2 will pass through the thermistors 5 and 6, flowing into the resistors 7 and 8 as overcurrent and to the ground through the varistors 3 and 4. In the case of an abnormally large voltage or current due, for example, to a contact with a power line, the thermistors 5 and 6 react to it and cut the current after a specified length of time has elapsed. Until the current is cut, however, the overcurrent passes through the thermistors 5 and 6 to flow into the resistors 7 and 8, escaping to the ground through the varistors 3 and 4.
With a protection device thus structured, thick-film resistors 7 and 8 with a relatively large allowable loss must be used because, as described above, there is a possibility of an overcurrent flowing into them. This means that fairly large resistors must be used, and that their anti-surge capability must be kept in mind when they are trimmed. If the trimming is effected by a straight cut, as usually done, the allowable power of surge which can be resisted by a thick-film resistor which has not been trimmed is 5 to 20 times greater than by a trimmed thick-film resistor, although it varies, depending upon the resistor material and the depth of the cut.